1. Field of the Invention
The present invention relates to apparatus for and a method of compression-molding plastic, and more particularly to apparatus for and a method of compression-molding a sheet molding compound (hereinafter referred to as SMC).
2. Description of the Related Art
SMC molded pieces have a beautiful appearance and exhibit excellent water resistance and chemical resistance and high mechanical strength. In addition, the SMC molded pieces are highly productive. Recently, SMC molded pieces have been used to fabricate automotive parts such as trunk lids, hoods and the like, as well as aero-parts, bath tubs, water tank panels, etc. In the applications mentioned above, the SMC is mostly compression-molded.
A mold generally includes a stationary die and a movable die, each die having an edge portion which when mated form a pinch-off section of the apparatus. These dies are made of steel, and a clearance of 0.01 to 0.10 mm is formed at the pinch-off section. After a predetermined amount of an SMC has been supplied, the SMC is compressionmolded under a pressure of 30 to 150 kgf/cm.sup.2 at 120.degree. to 180.degree. C. At this time, the SMC partially flows out through the clearance at the pinch-off section to the outside of the dies, thus forming flashes. The flashes which are unnecessary portions of the molded pieces, are removed by suitable means after compression molding. Such flash removal requires extra labor and time, lowering the productivity of the compression molding process. When the molded pieces are to be taken out from the mold, the flashes remain stuck to the apparatus at the pinch-off section, particularly when the molded pieces are thin at the end portions thereof, and as a result, the molded pieces are pulled by the flashes which still stick to the dies. This may cause the end portions of the molded pieces to be damaged, resulting in a defective product. This contributes to a loss in productivity.
Molded pieces to be used as automotive parts are mostly painted. Pinholes, porosities and the like in the molded pieces are exposed at those portions of the molded pieces from which flashes have been removed. This produces pinholes, craters and the like on the coated films, causing the appearance of the molded pieces to be considerably unsightly.
To prevent flashes from being produced, it is necessary to make as small as possible the clearance at the pinch-off section. However, if the clearance is too small in a steel die in which the edge portions forming the pinch-off section are steel, the edge portions interfere with each other due to, for example, unequal thermal expansion of the dies. This involves the likelihood of the edge portions becoming partially damaged. Thus, the clearance cannot be made small enough to prevent flashes from being produced.